Pigmentation due to spots, freckles, and the like emerges through formation of melanin pigment in epidermal chromocytes and abnormal deposition thereof in epidermis caused by ultraviolet exposure and the like. Melanin pigment is synthesized by metabolism of L-tyrosine as one of amino acids into L-dopa and then L-dopaquinone by the action of tyrosinase as an oxidative enzyme thereof and subsequent various pathways (cf. e.g., Fine Chemicals, issue on Mar. 15, 1999, “Special Topic: Bihakuzai no Kaihatsu to Seihin Tenkai (Development and Products of Whitening Agents)”; and Fragrance Journal, issue on September 1997, “Special Topic: Saikin no Bihakuzai no Kenkyu Kaihatsu Doko (Recent Trend of Researches and Development of Whitening Agent)”). Thus, in order to prevent pigmentation due to spots, freckles, and the like, it is important to inhibit activity of tyrosinase which plays an important role for melanin pigment synthesis.
Hitherto, for preventing and improving spots, freckles, and the like, there have been used pharmaceutical agents such as placenta extracts, vitamin C, vitamin C derivatives, kojic acid, and arbutin. However, these agents have resulted in no sufficient effect. Moreover, in Europe and the United States, hydroquinone has been used for the purpose of decoloration of dye freckles but use thereof is limited because of problems in safety. Furthermore, recently, a possibility of carcinogenicity is also pointed out for kojic acid and thus becomes a problem.
On the other hand, shogaol and gingerol are components of ginger extracts and they are known to have, for example, a blood cirdulation-facilitating effect (JP-A-6-183959), a body odor-preventing effect (U.S. Pat. No. 6,264,928), an antioxidation effect (H. Kikuzaki, N. Nakatani, “Antioxidant Effects of Some Ginger Constituents”, J. Food Sci., Vol. 58, No. 6, 1407-1410 (1993)), a moisturizing effect (supervising editor: Masato Suzuki, “Atarashii Keshohin Kinousozai (Novel Cosmetic Functional materials) 300, first volume”, pp. 311-312, CMC Publishing, 2002), and the like effects. Moreover, metabolic routes of shogaol and gingerol have been also researched and structures of metabolites thereof have been reported (H. Takahashi and other three authors, Phytochemistry, Vol. 34, 1497-1500 (1993) and S. S. Lee, Arch. Pharm. Res., Vol. 18, 136-137 (1995)).
The present inventors had investigated methods capable of mass production of shogaol and they developed an industrial production process of a specific shogaol as a target. Thus, they applied a patent application (Japanese Patent Application No. 2003-327574) on the specific shogaol and the process for producing the same, and simultaneously, they reported that shogaol has a property of inhibiting activity of tyrosinase. However, since the shogaol obtained by the production process is insufficient in water solubility, there is a case that it is difficult to apply to human and hence it is desired to develop a compound which is more soluble in water.